Tissues revised

Fundamentals of Tissue and Histology

• Tissue Definitions and Scope:

  • Tissue: Defined as a group of cells possessing similar structure and function.

  • Histology: The scientific study of tissues.

  • Major Body Tissue Types:

  • Epithelial Tissue

  • Connective Tissue

  • Muscular Tissue

  • Nervous Tissue

Epithelial Tissue

• General Overview and Characteristics:

  • Lines and covers body surfaces, organ surfaces, and body cavities.

  • Composed of layers of closely spaced cells that form glands and serve functions in protection, secretion, and absorption.

  • Examples include the epidermis, inner lining of the digestive tract, liver, and other glands.

  • Key Characteristics:

  • Avascular: Lacks or completely lacks blood vessels.

  • Basal Surface Attachment: Attaches at the basal surface, typically via a basement membrane.

  • Regeneration: High capacity for regeneration.

  • Cellularity: Composed almost entirely of cells with very little extracellular material.

  • Exposed Surface: Possesses a free (apical) surface exposed to body exterior or cavities.

  • Ubiquity: Found virtually everywhere both inside and outside the body.

  • Specialized Components: Composed of specialized cells and matrix.

• Primary Functions of Epithelial Tissue:

  • Protection: Protects underlying structures from physical or chemical damage (e.g., outer layer of skin/epidermis, oral mucosa).

  • Barrier: Acts as a protective barrier (e.g., outer layer of skin).

  • Permits Substance Passage: Allows movement of substances across tissue boundaries (e.g., alveolar epithelium in the lungs).

  • Secretion: Secretes products (e.g., mucous glands, sweat glands; all glands consist of epithelial tissue).

  • Absorption: Absorbs materials (e.g., epithelial lining of the small and large intestines).

  • Excretion: Excretes metabolic waste products such as urea (e.g., sweat glands).

• Epithelial Classification Based on Function:

  • Mucous Membrane: Secretes mucus, digestive enzymes, and bile salts; absorbs nutrients and offers protection.

  • Glandular Epithelium: Forms the functional units of glands.

  • Endothelium: Lines the internal surfaces of blood vessels and lymphatic vessels.

  • Mesothelium (Serous Membrane): Lines the major closed cavities of the body that do not open directly to the outside (pericardial, pleural, and peritoneal cavities).

• Structural Classification of Epithelial Tissue:

• Classification by Number of Cell Layers:

  • Simple Epithelium: Single layer of cells.

  • Stratified Epithelium: More than one cell layer thick.

  • Pseudostratified Epithelium: Single layer of cells, but nuclei at varying heights make it appear to have two or more layers.

  • Transitional Epithelium: Specialized stratified epithelium capable of significant stretching.

• Classification by Cell Shape:

  • Squamous: Flat and thin cells.

  • Cuboidal: Cube-like cells (equal height and width).

  • Columnar: Tall and thin cells.

  • Specific Types of Epithelia:

• Simple Squamous Epithelium:

* **Structure**: Single layer of flat, often hexagonal cells. Nuclei protrude as bumps in cross-sectional views due to extreme cell thinness.
* **Functions**: Diffusion, filtration, minor secretion, and protection against friction.
* **Locations**: Air sacs (alveoli) of lungs, inner surface of eardrums, endothelial lining of blood vessels, heart, and lymphatic vessels, mesothelial lining of serous membranes (pericardial, pleural, peritoneal cavities), portions of kidney tubules, and small ducts.

• Simple Cuboidal Epithelium:

* **Structure**: Single layer of cube-shaped cells. Apical surfaces may feature microvilli (kidney tubules) or cilia (terminal bronchioles).
* **Functions**:
  * Active transport and facilitated diffusion leading to secretion and absorption by kidney tubule cells.
  * Secretion by cells of glands and choroid plexuses.
  * Movement of particle-laden mucus out of terminal bronchioles via ciliated cells.
* **Locations**: Choroid plexuses of the brain, glands and their ducts, kidney tubules, lining of terminal bronchioles of lungs, and surfaces of the ovaries.

• Simple Columnar Epithelium:

* **Structure**: Single layer of tall, narrow cells. Some cells feature cilia (lung bronchioles, auditory tubes, uterine tubes, uterus) or microvilli (intestines).
* **Functions**:
  * Absorption by intestinal cells.
  * Movement of foreign particles out of lung bronchioles via ciliated cells.
  * Assists in propulsion of oocytes through uterine tubes via ciliated cells.
  * Secretion by glands, stomach, and intestinal cells.
* **Locations**: Auditory tubes, bile ducts, bronchioles of lungs, gallbladder, glands and select ducts, intestines, stomach, uterine tubes, uterus, and brain ventricles.

• Pseudostratified Columnar Epithelium:

* **Structure**: Single cell layer where all cells attach to the basement membrane, but only some reach the free surface. Nuclei are located at varying levels, giving a stratified appearance. Almost always ciliated and interspersed with mucus-secreting goblet cells.
* **Functions**:
  * Synthesize and secrete mucus onto the free surface.
  * Move mucus or fluid containing foreign particles across the surface and out of respiratory passages.
* **Locations**: Auditory tubes, bronchi of lungs, lining of nasal cavity, nasal sinuses, pharynx, and trachea.

• Stratified Squamous Epithelium:

* **Structure**: Multiple layers of cells that are cuboidal in basal layers and become progressively flattened toward the free surface.
  * **Nonkeratinized (Moist)**: Surface cells retain nuclei and cytoplasm.
  * **Keratinized**: Surface cell cytoplasm is replaced by the protein keratin; cells become nonviable.
* **Functions**: Protection against abrasion, barrier against infection, reduction of water loss.
* **Locations**:
  * *Keratinized*: Skin (epidermis).
  * *Nonkeratinized*: Cornea, mouth, throat, larynx, esophagus, anus, vagina, and inferior urethra.

• Stratified Cuboidal Epithelium:

* **Structure**: Multiple layers of somewhat cube-shaped cells.
* **Functions**: Absorption, protection against infection, secretion.
* **Locations**: Ovarian follicles, salivary gland ducts, and sweat gland ducts.

• Stratified Columnar Epithelium:

* **Structure**: Multiple cell layers with tall, thin surface cells resting on deeper cuboidal layers. Cells are ciliated in the larynx.
* **Functions**: Protection and secretion.
* **Locations**: Larynx, mammary gland ducts, and a portion of the male urethra.

• Transitional Epithelium:

* **Structure**: Cells appear cuboidal or organ-shaped when unstretched, but flatten into a squamous appearance when fluid distends the organ or tube.
* **Functions**: Accommodates volume fluctuations of fluid in organs or tubes; protects underlying structures from caustic effects of urine.
* **Locations**: Lining of urinary bladder, superior urethra, and ureters.

• Glandular Epithelium & Exocrine Classification:

  • Gland Definition: Single cell or multicellular structure that secretes substances onto a surface, into a cavity, or directly into blood.

  • Endocrine Glands: Ductless glands that secrete hormones directly into the bloodstream.

  • Exocrine Glands: Duct-containing glands that release secretions onto a free surface or into a body cavity (e.g., sweat glands, mammary glands).

  • Structural Classifications of Exocrine Glands:

  • Simple: Contains a single, unbranched duct.

  • Compound: Contains ducts that branch repeatedly.

  • Tubules: Secretory units end in small tubes.

  • Acini: Secretory units end in saclike structures.

  • Alveoli: Secretory units end in hollow sacs.

  • Classification by Secretion Mode:

  • Merocrine: Exocytosis without any loss of cellular material (e.g., sweat glands).

  • Apocrine: Secretory products gather at the cell apex, and that portion of the cell pinches off (e.g., mammary glands).

  • Holocrine: Secretory product accumulates within the cell, and the entire cell dies and is shed (e.g., sebaceous glands).

Connective Tissue

• General Characteristics and Functions:

  • Protects, supports, and binds body tissues together.

  • Consists of cells separated from one another by an abundant extracellular matrix.

  • Typically exhibits greater matrix volume than cellular volume.

  • Specialized to support and protect organs and connect tissues/organs.

  • Examples include blood, bone, cartilage, ligaments, lymph, pulp, and tendons.

  • Primary Functions:

  • Support: Bones, muscles, nerves, blood vessels, fat, skin, cartilage.

  • Nourishment: Blood, synovial membranes.

  • Transportation: Blood.

  • Connection: Tendons (muscle to bone), ligaments (bone to bone).

  • Movement: Muscles, bones.

  • Protection: Bones, blood cells.

  • Insulation: Adipose tissues.

  • Storage: Bone, adipose tissue.

  • Attachment and Separation: Attaches skin to underlying muscle layers.

• Extracellular Matrix Components:

  • **Protein Fib